I can see where you're coming from, that an artist's or album's influence at a certain time certainly biases its reputation (Elvis Presley, anyone?), but one could make an argument that, since the Beatles and Bob Dylan consistantly capture the imagination of multiple generations, their music has some sort of inherent staying power. (I'm playing devil's advocate here, but I'm really interested in responses to this).
Still, like jackjammer said, the problem is that some critics do seem to judge from a detached base, which ruins the purpose of music. No one listens to an album because it's "good"; we listen to music because we can relate to it.
If it is possible to objectively judge music (and I don't think it is), that exercise has no purpose.
The only music reviews worth reading are ones that focus on the author's personal experience, imo (parts of Greil Marcus' "Stranded," this book called "This Is Uncool" by Gary Mulholland, etc). The reviews that try to place music in a social context and analyze its political effects (Dave Marsh, Jon Landau, etc) annoy me to no end.
How do some people have more credible opinions (and how do you tell?)? Perhaps experience (i.e., listening to lots of music), but as you said, it's still subjective (and listening to a lot of music would only tend to further bias one's opinions, I would think).
